Preheat oven to 375 degrees F.
Brush 4 large baking sheets well with oil. Arrange the potato slices in one layer on the sheets. Brush with the remaining oil, and top with another baking sheet to prevent curling and bake until golden, 30 to 35 minutes, switching the sheets halfway through baking.
Transfer with a spatula to cooling racks. Sprinkle with the Essence and salt, and set aside.
In a large, heavy skillet, melt the butter over medium-high heat. Add the shallots and cook, stirring, for 1 minute. Add the mushrooms, salt and pepper, and cook, stirring, until soft and most of the mushroom liquid is evaporated, about 8 minutes.
Add the stock, cream, and thyme, and simmer until the liquid has reduced by 50 percent in volume, about 5 minutes. Remove from the heat and stir in the truffle oil.
Place 2 potato slices on each of 4 serving plates. Spoon a generous tablespoon of mushroom filling on top, topped with a teaspoon of grated cheese. Continue layering the napoleon so that you have 3 layers of potatoes and 3 layers of mushrooms in all, ending with mushrooms on top. Garnish the top of each napoleon with a sprinkle of cheese and 3 slices of truffle, if desired. Arrange the chives around the outer edge of the plate and serve.
Yield: 8 servings